CSC362.01, Class 4: Sample Source and Target Languages

Admin:
* What subject to use for your email
* When is homework due?  At the start of class!
* Prospective visiting faculty coming in the next few weeks.  Please
  attend their talks.
* Summer research talk next Wednesday.
* Putnam talk today.
* Cool convo Thursday.  Extra credit for attending.
* Watch Sam suffer on a panel Thursday evening.
* Lots of this stuff is online in the Examples directory.

Overview:
* PAL, A pseduo-assembly language (intermediate rep)
* STUPID, A simple high-level language
* Lexing STUPID
* Parsing STUPID

Normal computer operation:
* Read next instruction from memory
* Decode the instruction
* Fetch some operands from memory
* Executes the operation
* Stores the results somewhere

Suppose we need to represent intermediate code for programs (not high level, not too low level), what should we do?
* A data structure in the language of your choice, rather than a textual intermediate representation
* Close to assembly language, so translation to "real assembly" language is "easy"
* Removes many of the complexities of real assembly language:
  * No restrictions on the number of registers
  * No restrictions on types of operands
  * Tree-like rather than linear (perhaps less parsing, matches translation; may be easier to find things)
  * Reasonable but limited set of operations

PAL is a set of Java classes
* Computer
* Instruction (interface)
  + .execute()
* Variable: Sources and destinations of values
  * Standard registers (Registers.pc, Registers.fp, Registers.bp)  
  * Temporaries: "Infinite" user-defined registers
	Variable x = new Temporary();
  * Memory locations (indexed by variables)
  * Offsets from memory locations
  * "Constants"

The instructions
* MOVE source -> destination
	Instruction foo = new Move(x,y);
* Arithmetic: [IF]Add, [IF]Sub, [IF]Mult, [IF]Div
* Jumps 
* Instruction Sequence
* Random other stuff: Labels, NoOp, Halt, [IF]Read, [IFS]Write, Conversion
Sample code